1. Field of the Invention
This invention relates to a multi-layer structure useful for packages, and more particularly relates to a multi-layer structure which is capable of retaining an oxygen barrier property satisfactorily in spite of aging, excellent in the ability to preserve wrapped contents such as foodstuffs and medicines, and useful for various packaging applications.
2. Description of the Prior Art
Oxygen barrier resins such as saponified ethylene-vinyl acetate copolymer (hereinafter referred to as "EVOH") have low oxygen permeability and yet high humidity dependency and, therefore, are liable to suffer from a notable increase of oxygen permeability under a highly humid atmosphere. Generally, packaging structures, therefore, are adapted to retain the oxygen barrier property thereof by laminating a protective layer, such as of polyethylene having a low water absorption or low moisture absorption on a layer of an oxygen barrier resin of high humidity dependency thereby precluding direct exposure of the oxygen barrier layer to water or moisture (as disclosed in Japanese Utility Model Application KOKAI (laid open to public inspection) No. 47(1972)-23,353 for example). Under the conditions of high humidity and high temperature as in the treatment in a retort for sterilization, however, the polyethylene itself protecting the oxygen barrier layer gains in the moisture absorption and notably increases the moisture permeability thereof and, as the result, the moisture permeating the protective layer passes into the oxygen barrier layer and seriously jeopardizes the oxygen barrier property of such packaging structures.
For the purpose of eliminating the drawback mentioned above, it has been proposed to repress the passage of permeating moisture into the oxygen barrier by incorporating a water-absorbing substance in the aforementioned protective layer or adhesive intermedite layer thereby relying on the layer incorporating therein this water-absorbing substance to absorb the permeating moisture en route to the oxygen barrier layer. The specification of Japanese Patent Application KOKAI No. 57(1982)-170,748, for example, discloses an idea of providing a layer incorporating therein a drying agent such as an inorganic compound like calcium chloride or a saccharide like sucrose for a laminated structure containing an oxygen barrier layer of such a water-sensitive polymer as EVOH thereby protecting the oxygen barrier layer of the water-sensitive polymer against moisture. In the multi-layer structure provided with such a drying agent-incorporating layer as mentioned above, however, despite the incorporation of the drying agent in the resin forming the inner and outer layers or in the adhesive layer, the drying agent is so deficient in water absorption property that it is actually difficult to produce a multi-layer structure possessing as high oxygen barrier property as expected unless the drying agent is used in a large amount. The incorporation of the drying agent in an excess amount in the resin forming the aforementioned layer for the purpose of improving the water absorbing effect entails a disadvantage that the characteristic properties such as, for example, transparency and physical strength which are possessed by the resins forming the component layers of the structure will be impaired.
The specifications of Japanese Patent Application KOKAI No. 58(1983)-98,251 and No. 61(1986)-11,339 disclose an idea of interposing between a gas barrier layer and inner and outer surface layers of moistureproof resin intermediate layers having highly water-absorbing resin particles dispersed in film-formable and particularly thermally formable resin. The highly water-absorbing resin excels the aforementioned drying agent in water absorbing ability. When the highly water-absorbing resin particles of the nature described above are dispersed in the polyolefinic resin used in the intermediate layers, they entail a disadvantage that the polyolefinic resin enveloping the dispersed particles shows substantially no discernible loss of hardness due to moisture absorption and possesses a very low capacity for swelling with absorbed water and are swelled with great difficulty, the highly water-absorbing resin particles dispersed therein are impeded from being swelled with absorbed water and, therefore, are restrained from manifesting the ability thereof to absorb water. Under the conditions of high humidity and high temperature generated as in the treatment using a retort, even when the multilayer structure is configurated as described above, it is difficult from the practical point of view to ensure perfect prevention of the oxygen barrier resin from absorbing water. In the multi-layer structure having the highly water-absorbing resin particles dispersed in the intermediate layers formed mainly of a polyolefinic resin, therefore, there is involved a problem that when the highly water-absorbing resin particles are relied on to absorb water from the oxygen barrier resin in a state wet with water already absorbed during the course of manufacture of the structure, the absorption of this water is effected indirectly through the medium of the hydrophobic polyolefin resin and the actual water absorption displayed by the resin particles is very inferior.
In the case of a multi-layer structure having the aforementioned highly water-absorbing resin particles dispersed in an adhesive layer of hydrophobic adhesive resin as disclosed in the specifications of Japanese Patent Application KOKAI No. 58(1983)-98,251 and No. 61(1986)-11,339, the structure shows a phenomenon of foaming in consequence of relief of pressure during the course of extrusion molding and suffers occurrence of countless fine bubbles distributed within the walls of the component layers. Consequently, the adhesive layer and further the oxygen barrier layer produce pinholes therein and, at times, fail to attain easily the object of enabling the structure to retain the oxygen barrier property. Further, the dispersion of the highly water-absorbing resin particles in the adhesive layers results in a decrease of interlaminar strength and the inclusion of the highly water-absorbing resin particles in the inner and outer adhesive layers entails a sacrifice of transparency and an increase of production cost.
The aforementioned prior arts invariably aim to prevent an oxygen barrier layer of water-sensitive resin such as EVOH from absorption of moisture and consequent adverse effects by laminating on the oxygen barrier layer or outer layer or adhesive layer made of moistureproof resin having a water-absorbing substance dispersed therein. They do not contemplate having the water-absorbing substance dispersed in the oxygen barrier layer itself.
U.S. Pat. No. 4,425,410 (filed Sept. 15, 1982) discloses a multi-layer structure which has calcium chloride, sodium chloride, sodium phosphate-di-basic, potassium carbonate, aluminum sulfate, magnesium sulfate, magnesium chloride, sodium nitrate, sodium sulfate, ammonium phosphate, or potassium bromide or sucrose dispersed as a drying agent in an oxyen barrier layer itself. U.S. Pat. No. 4,464,443 (filed Oct. 3, 1983) discloses a mutli-layer structure having an ammonium chloride drying agent dispersed in an oxygen barrier layer thereof. The drying agent used therein as described above, unlike the hydrophilic macromolecular compound which swells in consequence of water absorption, absorbs water from the environment only to form a hydrate or convert itself into an aqueous solution and, therefore, possesses an extremely low water-absorbing property as compared with the hydrophilic macromolecular compound. Once this drying agent is saturated with absorbed water, it is no longer capable of absorbing water. When the multi-layer structure which has this drying agent dispersed in the oxygen barrier layer is used under the conditions of high humidity and high temperature, particularly for a long time, it retains the oxygen barrier proper appreciably only with difficulty. Particularly when the drying agent is incorporated in the oxygen barrier layer in a large amount for the purpose of enhancing the water-absorbing effect, there ensues exactly the same problem as involved when the incorporation is made in the adhesive layer, i.e. the problem that the transparency, physical properties, etc. of the oxygen barrier layer are impaired.